Blood capillaries discontinuous

Capillaries are the exchange vessels of the body. They have structural variations to allow different levels of metabolic exchange (of exogenous and endogenous substances) between blood and the surrounding tissues. The structure of the walls varies depending on their resident tissue. There are three major types of blood capillaries continuous fenestrated and sinusoidal (discontinuous) [1] ... 

Figure 5.1 Schematic illustration of the structure of the wall of different classes of blood capillaries. (1) Continuous capillary (as found in the general circulation). The endothelium is continuous with tight junctions between adjacent endothelial cells. The subendothehal basement membrane is also continuous. (2) Fenestrated capillary (as found in exocrine glands and the pancreas). The endothelium exhibits a series of fenestrae which are sealed by a membranous diaphragm. The subendothehal basement membrane is continuous. (3) Discontinuous (sinusoidal) capillary (as found in the liver, spleen and bone marrow). The overlying endothelium contains numerous gaps of varying size. The subendothehal basement is either absent (hver) or present as a fragmented interrupted structure (spleen, bone marrow)...

Figure 2.16b shows the structure of the glomerular vessel wall. The interior is covered by endothelial cells with multiple holes ( fenestrations ). The podocytes (= foot cells ) form a likewise discontinuous outer layer. Between them is an acellular basal membrane, consisting of proteins and proteoglycans, which has the smallest pore diameter of all three and therefore, as in any the capillaries found elsewhere in the body, represents the effective filter layer. The filter has a cut-off size of very few nanometers, so that most protein molecules will be retained. Salt ions and small molecules - if they are not protein-bound - will be filtrated. The amount of filtrate produced is about 1501 per day in a healthy adult this corresponds to about 1/10 of the blood plasma volume that passes the kidneys. 

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